As is well known, prospecting for minerals, hydrocarbons such as oil and gas, and other natural resources of commercial value is economically an extremely important activity. For various reasons those wishing to extract resources from below the surface of the ground or the floor of an ocean need to acquire as much information as possible about both the potential commercial worth of the natural resources in a geological formation and also any difficulties that may arise in extracting them to surface locations at which they may be used.
Techniques of logging of subterranean formations have developed for the purpose of establishing, with as much accuracy as possible, information as outlined above both before extraction activities commence and also while they are taking place.
During drilling operations, a surface hole is drilled to a location near where the prospective resource is located. The boreholes used for the purpose explained above may extend for several thousands or tens of thousands of meters from a surface location.
A drill pipe is typically a hollow, thick-walled, steel piping used on drilling rigs to facilitate the drilling of a borehole/wellbore. The pipe consists of numerous pieces screwed one to another. Each piece of drill pipe is approximately ten meters long. Usually a piece has external male threads at one end and female threads in the internal diameter of the other end. The male threads of one piece engage with the female threads of another piece, thereby joining the pieces of drill pipe together.
Normally while borehole drilling is carried out, a string of compound pieces of drill pipe is used in order to drive a rotatable drill bit mounted at the end of the pipe in contact with the ground.
As the drill bit works its way down into the ground and the borehole length increases, the drill pipe is sequentially extended by adding a new piece of drill pipe at a time to its upper end. As a result long lengths of drill pipe may be inserted into boreholes as they are formed.
Broadly stated, logging involves inserting a logging tool, also known as a sonde, into a borehole or other feature penetrating a formation under investigation; and using the sonde to energise the material of the rock, etc., surrounding the borehole in some way. Passage of the energy through the rock alters its character. The sonde or another tool associated with it that is capable of detecting energy is intended then to receive emitted energy that has passed through the various components in the rock before being recorded by the logging tool.
Typically the logging tool is formed as an elongated, rigid cylinder that may be e.g. one to five meters long.
A wireline, as is well known in the art, is an armoured cable that may be used for the purposes of lowering a logging tool into the borehole, or supporting the tool while it is being withdrawn upwardly along a borehole or well during logging. The logging tool is located at the end of the wireline. Logging measurements are in one known method taken by lowering the wireline to the prescribed depth and then raising it out of the well while operating the logging tool. Wireline is capable of electronically telemetering data from various types of logging tool from downhole to surface locations; and also of sending electronic commands to connected downhole equipment. In some situations however it is not possible or desirable to maintain the wireline connected to the logging tool following deployment of the latter.
Wireline drop-off is a conveyance system that allows for openhole data acquisition while tripping (i.e. the act of pulling the drill pipe out of the hole or replacing it in the hole). In this conveyance technique shown partially in FIGS. 1-2, a logging tool 10′ having a memory function is conveyed down-hole by wireline 30 through drill pipe 40 and hangs into the openhole on a no-go at the bottom of the drill pipe 40.
When drilling has reached total depth (the planned end of the well measured by the length of pipe required to reach the bottom), for example, the wireline 30 is released into the drill pipe 40. Typically there is a landing ring 42 in the internal wall of the drill pipe 40, located near the mouth of the final (i.e. most downhole) piece of drill pipe 40, which receives a landing collar 22 located on and protruding outwardly from the tool 10′. The engagement of the landing ring 42 and collar 22 secures the tool 10′ and pipe 40 one to another. When this engagement has occurred, the wireline 30 is removed from the well.
The result of this sequence is that part of the logging tool 10′ protrudes beyond the end of the drill pipe 40 and therefore is exposed in a way that permits logging of the formation. A further part of the logging tool 10′ remains inside the drill pipe 40 and defines the described landing collar connection to the drill pipe 40.
To withdraw the drill pipe 40, the pieces of drill pipe 40 at the surface are unscrewed one by one from each other to separate them. As a result the drill pipe 40 is gradually withdrawn from the borehole B. A dropped-off logging tool 10′, therefore, moves towards the surface with the pipe 40, taking records (well logs) of the formation F along the way.
Each time a piece of drill pipe 40 is to be removed from the upper end, the withdrawal operation is interrupted while unscrewing of the drill pipe 40 takes place.
Because drillers often use high-torque couplings to prevent the pieces of drill pipe 40 from coming unscrewed downhole, the pieces may become stuck to each other making it difficult for them to be separated.
The protruding nature of the landing collar 22 on the tool 10′ may prevent it from entering drill pipes 40 having a small internal diameter.
The current wireline drop-off technique only enables the tool 10′ to log the openhole beyond the landing collar 22 into which the tool 10′ protrudes. This limits the length of the openhole where formation data could be acquired. It would however be desirable to log openhole that is well beyond the end of the drill pipe 40.
Moreover, if it is realised that part-way through the drill pipe 40 withdrawal process some parts of the formation F have not been logged, the tool 10′ cannot be disengaged from the drill pipe 40 to be lowered while the pipe 40 is downhole. The drill pipe 40 must be removed and the whole operation needs to be repeated to obtain any desired missing logs. This again adds to operation costs.
Typically during the drop-off deployment using the techniques of FIGS. 1-2, the logging tool 10′ is conveyed downhole inside the drill pipe 40 connected at its uphole end to wireline 30, the nature and functioning of which are known in the art. Once the tool 10′ in the prior art arrangement reaches the position shown in FIG. 2, the collar 22 lands on the landing ring 42 with part of the tool 10′ protruding in a downhole direction beyond the end of the drill pipe 40.
The tool 10′ is secured in this position and the wireline (30) disconnected from its uphole end. The wireline 30 is then withdrawn entirely from the drill pipe 40. Step by step withdrawal of the drill pipe 40 as described above then permits logging of the borehole B to take place, using on-board battery and memory M of the tool 10′.
However, on occasions, the tool 10′ may fail to log sections of the borehole B. Using the prior art arrangement of FIGS. 1-2, it is possible to log such missed regions only by sending the wireline 30 all the way back to the tool 10′, reattaching the wireline 30 to the tool 10′ (using known wireline latching parts), withdrawing the tool 10′ to a surface location and then adding pieces of drill pipe 40 until the end of the drill pipe 40 coincides with the un-logged part of the borehole B.
Thereafter, it is necessary to repeat the tool drop-off steps in order to deploy the tool 10′, whereafter it becomes possible to log the missed portions.
This sequence is so time-consuming as to render it uneconomic in many cases. Moreover, the described technique results in the tool 10′ passing some parts of the borehole B twice during repeated runs. This is inefficient.
The subject matter of the present disclosure aims to solves one or more problems that exist in the prior art.